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Introduction

Preterm delivery is defined as delivery prior to 37 completed weeks of gestation. The shorter the gestational age of an infant, the more likely it is for that infant to suffer adverse effects. Preterm birth along with low birthweight is the second leading cause of infant death (see the Infant Mortality indicator), and preterm infants are at higher risk for health and developmental problems (IOM, 2006).

The determinants of preterm births are not fully known and the causes are often multi-factorial. Maternal high-risk conditions (e.g., infertility problems, vaginal spotting, inadequate maternal weight gain), previous history, socioeconomic status, smoking, alcohol consumption, and multiple gestation pregnancy are some of the known risk factors for preterm delivery. Environmental contaminants (e.g., lead, environmental tobacco smoke, air pollution) continue to be studied to better understand the strength of the associations with preterm delivery (CDC, 2012a).

This indicator presents the proportion of U.S. infants born prior to 37 weeks of gestation, based on natality data reported to the National Vital Statistics System (NVSS). The NVSS registers virtually all deaths and births nationwide, with data coverage from 1933 to 2010 and from all 50 states and the District of Columbia. The data presented here on preterm delivery were based on singleton births only. This was done to eliminate the effect of multiple births. The data are presented across three maternal age groups (under 20 years, 20-39 years, and 40 years and older).

What the Data Show

The proportion of all infants defined as preterm rose more than 20 percent from 1990 to 2006, but declined 6 percent by 2010 (NCHS, 2012). A small overall increase in preterm deliveries of singleton infants has been observed from 1995 (9.8 percent) to 2010 (10.3 percent). The largest percent increase between 1995 and 2010 has occurred among mothers in the 40 and over age group, with the percent of preterm births ranging from 12.0 percent (1995 and 1996) to 13.9 percent (2006 and 2007) and then decreasing to 13.3 percent by 2010. The next largest percent increase was observed in the 20-39 year old maternal group, ranging from 9.2 percent (1996) to a high of 10.7 percent (2006) and then decreasing to 9.9 percent by 2010. A smaller change (1 percent or less) was observed among those under 20 years of age, with an overall increase from 1995 (13.1 percent) to 2008 (13.2 percent), and then decreasing in 2009 and 2010 (12.8 percent) (Exhibit 1).

In 1995, the percent of singleton preterm births was highest among black mothers (16.4 percent), followed by American Indian (11.5 percent), Asian/Pacific Islander (9.2 percent), and white (8.5 percent) mothers. From 1995 to 2010 preterm delivery among these mothers has shifted up and down slightly, with small percent differences (2 percent or less) across races over time (Exhibit 1). Specifically, from 1995 to 2010 the percent of preterm births ranged from 8.5 percent (1995 and 1996) to 10.1 percent (2005 and 2006) for white mothers, 15.1 percent (2010) to 16.4 percent (1995) for black mothers, 11.0 percent (1996) to 13.0 percent (2006) for American Indian mothers, and 8.9 percent (2000) to 9.6 percent (2006 and 2007) for Asian/Pacific Islander mothers. Preterm delivery for Hispanic mothers ranged from 10.1 percent (1995 and 1996) to 11.2 percent (2006 and 2007), compared to 9.7 (1996) and 11.1 (2006) percent for non-Hispanic mothers between 1995 and 2010. From 1995 to 2010, the percent of preterm births was higher among Hispanic mothers except in 2005 when the percent was 11.0 percent for both Hispanic and non-Hispanic mothers. In the most recent year (2010), the percent of preterm births was 10.7 percent for Hispanic mothers and 10.2 percent for non-Hispanic mothers.

Limitations

The primary measure used to determine the gestational age of the newborn is the interval between the first day of the mother's last normal menstrual period (LMP) and the date of birth. This measurement is subject to error for reasons such as imperfect maternal recall or misidentification of the LMP because of postconception bleeding, delayed ovulation, or intervening early miscarriage. When the LMP and date of birth are clearly inconsistent with the infant's birthweight or plurality, then a “clinical estimate of gestation” is used. Problems with reporting gestational age persist and may occur more frequently among some subpopulations and among births with shorter gestations (CDC, 2010).

Data Sources

The data used for this indicator were public-use natality data (1995-2010) obtained from the Centers for Disease Control and Prevention’s (CDC’s) National Center for Health Statistics (NCHS), Division of Vital Statistics, accessed via CDC’s NCHS VitalStats (CDC, 2012b) available at http://www.cdc.gov/nchs/vitalstats.htm.

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